The predictor of severe disease in respiratory syncytial virus (RSV) infections is the replication of RSV in activated T lymphocytes with an enhanced lymphorproliferative response and resultant production of inflammatory cytokines (IFNy, IL-4, IL-5, and IL-10) that cause lung damage. Our specific aims are: 1. Determine the predominant lymphocyte cell type for RSV replication, in young children with severe LR1. 2. Determine functional correlates of severity (T cell, cytokine, chemokines). Our strategy for testing this hypothesis is to obtain lymphocytes by bronchoalveolar lavage (BAL) in intubated children with and without RSV infection; to show viral replication in activated T-Lymphocytes by fluorescence activated cell sorter (FACS) scan and viral culture; to measure functional correlates by measuring H3 thymidine uptake in lymphocytes and measurement of the cytokines produced in BAL; and to correlate the number and type of lymphocytes infected with the levels of cytokines/chemokines produced.